1890.] NATURAL SCIENCES OF PHILADELPHIA. 387 



where for the origin of the so-called ' anal' plates." To this we re- 

 ply, there are " interradial plates" in the Fistulata as well as in the 

 Camerata, but they are in the former interambulacral and not in- 

 terbrachial, owing to the fact that the arms are free from the radi- 

 als. All plates between the rays, those between the ambulacra, and 

 all the plates of the ventral sac, those supporting it, and the plates 

 of the anal tube, are parts of the same element. 



The symmetry of the Crinoids, as a rule, is bilateral, and the anal 

 area occupies the median axis. The asymmetry, which occurs in so 

 many Fistulata and certain Ichthyocrinidae, is caused by irregu- 

 larities in the radials. Wherever these have attained a regular form, 

 the plate X takes its median position, and the plates of the ventral 

 tube also are arranged on a strictly bilateral plan. Whether the 

 synmietrical form, as represented in tlie Silurian Cyathocrinidae, was 

 evolved from the asymmetrical form, we are unable to ascertain. It 

 is possible that the two had a common symmetrical ancestor, but it 

 seems to us more probable that all were at first asymmetrical, and 

 that the lower section of the posterior radial became early resorbed 

 in some cases. Against the former theory it may be said that in the 

 Lower Silurian Fistulata, so far as known without exception, the 

 right posterior radial is compound, and that we find the symmetrical 

 form along with the other in the same families ; against the latter, 

 that the symmetrical form is already well represented in the Upper 

 Silurian. 



We have made no reference to the Calceocrinidae, as we have not 

 at present the material to study the older forms, but we are quite 

 certain that their structure is in conformity with that of the other 

 Fistulata. The statement, that " Castrocrinus shows a series of anal 

 plates supported by the first costal of the left posterior radius, which 

 is exactly comparable to the arm-bearing costal of the right pos- 

 terior radius," leads us to suspect that there is also here a misinter- 

 pretation of the plates. 



In the Ichthyocrinidae the anal structure resembles that of the 

 Fistulata. In some ot them both anal plates are represented, 

 in others only the plate X ; while in the genus Ichthyocrinus, both 

 plates are absent. The following table shows the distribution of the 

 anal plates in the different genera, and the representation of inter- 

 brachial plates. 



